PCCP View Article Online
Published on 30 May 2014. Downloaded by University of Tasmania on 31/08/2014 14:55:42.
PAPER
Cite this: Phys. Chem. Chem. Phys., 2014, 16, 15709
View Journal | View Issue
The crystal plane effect on the peroxidase-like catalytic properties of Co3O4 nanomaterials† Jianshuai Mu, Li Zhang, Guangyu Zhao and Yan Wang* Nanomaterials as enzyme mimics have received considerable attention as they can overcome some serious disadvantages associated with the natural enzymes. In recently developed Co3O4 nanoparticles as peroxidase mimics, the influence of the crystal plane on the catalytic performance has not been demonstrated. In order to better understand their crystal plane-dependent catalysis, the present study was initiated using three different Co3O4 nanomaterials, nanoplates, nanorods and nanocubes, as model systems. According to HRTEM, the predominantly exposed planes of nanoplates, nanorods and nanocubes are {112}, {110} and {100} planes, respectively. The catalytic activities were explored by using H2O2 and different organic substrates as the substrates of peroxidase mimics, and were investigated in-depth by steady-state kinetics and electrochemistry methods in depth. The results show that the peroxidase-like activity increases from nanocubes to nanoplates, via nanorods. The effect of external conditions such as pH and temperature on the three nanomaterials is the same, which indicates that the difference in their catalytic activities originates from their different shapes. The peroxidase-like catalytic activities of Co3O4 nanomaterials are crystal plane-dependent and follow the order: {112} c {110} 4 {100}. The three crystal
Received 27th March 2014, Accepted 27th May 2014
planes have different arrangements of surface atoms, thus exhibiting different abilities of electron transfer,
DOI: 10.1039/c4cp01326c
which induce their different peroxidase-like catalytic activities. This investigation clarifies that the
www.rsc.org/pccp
that Co3O4 nanomaterials can serve as catalyst models for designing other catalysts.
peroxidase-like activity of Co3O4 nanomaterials can be enhanced by shape control. These findings show
1. Introduction Artificial enzymes are a rapidly developing field due to the intrinsic drawbacks associated with the natural enzymes, such as low stability, undesirable sensitivity of catalytic activity to environmental conditions, requirement of special storage conditions and high cost.1,2 Several years ago, Fe3O4 nanoparticles were discovered to have the peroxidase-like activity and could be used as peroxidase mimics to catalyze and detect some molecules.3 This gave rise to new possibilities in the field of nanomaterial-based artificial enzymes. With the developments in the field, other nanomaterials have also been evaluated as enzyme mimics.4–13 The nanomaterialbased enzyme mimics take advantage of low cost, high stability and tunability of catalytic activities and can be used in bioassays and medical diagnostics.14 Nanocatalysis is an explosively growing topic because of its considerable contributions to both materials science and heterogeneous catalysis. Researchers have long been seeking catalysts with small nanometer-sized particles, because the
Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin 150001, China. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4cp01326c
This journal is © the Owner Societies 2014
nanoscale catalysts should offer higher surface areas, lowcoordinated sites, and surface vacancies, which are responsible for the higher catalytic activity.15–17 It has been shown that the catalytic performance of a catalyst is associated with not only the size but also the shape. By varying the shape of the crystals, the exposed crystal planes are affected, thus influencing the catalytic performance. For instance, the orders of magnitude variations in the rate of ammonia synthesis were reported on different planes of iron surfaces (the contribution recognized with a Nobel Prize in 2007).18,19 The planes of technical catalysts are poorly defined which may lead to nonideal catalytic performance. It also hampers the understanding of the catalytic nature, which is one of the major challenges in heterogeneous catalysis. Therefore shapecontrolled nanocatalysts with well-defined planes are novel simple systems in order to facilitate the design and fabrication of highly efficient catalysts.20 During the past decade, the rapid development of material science has offered an opportunity to prepare metal and/ or metal oxide nanomaterials with specific planes exposed.21–23 It is now time to discuss whether there are new ideas regarding shape that can lead catalysis research into new directions. Several recent studies have demonstrated the importance of a well-defined shape/ plane in enhancing catalytic performance.24–34 Therefore, shapecontrollable synthesis of nanocatalysts is an excellent way of obtaining the next generation high-performance catalysts.
Phys. Chem. Chem. Phys., 2014, 16, 15709--15716 | 15709
View Article Online
Published on 30 May 2014. Downloaded by University of Tasmania on 31/08/2014 14:55:42.
Paper
PCCP
Not long ago, we firstly found that the Co3O4 nanoparticles exhibited the peroxidase-like catalytic activity.8 However, their peroxidase-like catalytic properties with different crystal planes have not been addressed. In order to understand this crystal plane effect, the Co3O4 nanomaterials with different shapes were chosen as model systems. Their different shapes exposed different crystal planes. The peroxidase-like catalytic performance of Co3O4 nanomaterials was studied using typical substrates, such as 3,3 0 ,5,5 0 -tetramethylbenzidine (TMB), o-phenylenediamine (OPD) and 3,3 0 -diaminobenzidine (DAB), in the presence of H2O2, and the steady-state kinetics of the three nanomaterials were also investigated. Then, a correlative investigation between the crystal planes of Co3O4 nanomaterials and their catalytic performance is presented. This work not only favours the deep understanding of the catalytic phenomenon, but also provides a potential way to develop more efficient catalysts for some applications.
2. Experimental section 2.1.
Materials
All chemicals used for the synthesis of Co3O4 nanomaterials were of analytical grade and used as received without further purification. CoCl26H2O, ammonium hydroxide, Co(NO3)26H2O, hexamethylenetetramine, sodium hydroxide, Co(CH3COO)24H2O, sodium carbonate, ethylene glycol and hydrogen peroxide were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). The peroxidase substrates including 3,30 ,5,50 -tetramethylbenzidine (TMB), o-phenylenediamine (OPD) and 3,3 0 -diaminobenzidine (DAB) were obtained from Sigma-Aldrich (St. Louis, USA). 2.2.
Preparation of Co3O4 nanomaterials
Co3O4 nanoplates were synthesized by a hydrothermal method according to the reported route.35 In a typical reaction process, for the synthesis of Co3O4 nanoplates, 1.3 g of Co(NO3)26H2O was dissolved in 100 mL of water, and mixed with 100 mL solution containing 0.6 g of hexamethylenetetramine (HMTA), under continuous stirring. A few drops of NaOH solution were then added into the above solution to obtain pH 10. After maintaining the pH, the solution was vigorously stirred for 2 h. Consequently, the solution was then loaded into the Teflonlined stainless steel autoclaves, sealed and heated up to 110 1C for 15 h. After completing the reaction, the autoclave was naturally allowed to cool at room-temperature and the products were washed with water, ethanol and acetone sequentially and dried at room temperature. The dried products were then calcined at 200 1C for 2 h. The Co3O4 nanorods were prepared according to the method reported in the literature.27 3.32 g of Co(CH3COO)24H2O was dissolved in 40 mL of ethylene glycol and the mixture was gradually heated to 160 1C. Under a continuous flow of nitrogen, 134 mL of aqueous 0.2 M sodium carbonate solution was then added at the rate of 1.1 mL min1 and the slurry was further aged for 1 h under vigorous stirring. After centrifugation and washing with water, the solid obtained was dried at
15710 | Phys. Chem. Chem. Phys., 2014, 16, 15709--15716
50 1C overnight under vacuum and then calcined at 450 1C for 4 h in air. The Co3O4 nanocubes were synthesized according to the literature.36 1 g of Co(NO3)26H2O was dissolved in 40 mL of water and mixed with 0.8 mL of 30 wt% hydrogen peroxide. And pH of the solution was maintained at 9 by adding ammonia. Then the reaction mixture was charged in a 50 mL capacity autoclave with Teflon liner followed by uniform heating at 180 1C for 12 h. After the reaction was completed, the autoclave was allowed to cool to room temperature naturally. The solid precipitate was collected by centrifugation, washed several times with water, and then dried at 80 1C in air. 2.3. Characterization of differently shaped Co3O4 nanomaterials The composition and phase of the synthesized product were identified by powder X-ray diffraction (XRD) on an X 0 Pert PRO X-ray diffractometer (PANalytical, Netherlands) using Cu Ka radiation (l = 1.5418 Å). The morphology and size of the product were examined by scanning electron microscopy (SEM) using a SU-8000 SEM (Hitachi, Japan) at an accelerating voltage of 15 kV. The structure and composition of the products were characterized by means of a high-resolution transmission electron microscope (HRTEM, Tecnai G2 F30, USA). FT-IR spectra were recorded on a Spectrum 100 Frontier IR Spectrometer (Perkin Elmer, USA). The N2 adsorption–desorption isotherms of the Co3O4 nanomaterials were measured on an ASAP 2020 Physisorption Analyzer (Micromeritics, USA) at 77 K, and the specific surface areas were obtained by the Brunauer– Emmett–Teller (BET) method. 2.4. Catalytic experiments and electrochemical measurements The peroxidase-like catalytic activities of the Co3O4 nanomaterials were examined in phosphate buffer (3 mL, 100 mM, pH 6.0) containing Co3O4 nanomaterials, in the presence of H2O2 and the organic substrate (TMB, OPD or DAB). The reactions were monitored in wavelength-scan mode or time-drive mode by using a Lambda 750 UV-vis-NIR spectrophotometer (Perkin Elmer, USA). The apparent steady-state reaction rates of three Co3O4 nanomaterials were deduced according to the initial linear range of the kinetic curves and the molar absorption coefficient of 39 000 M1 cm1 for TMB-derived oxidation products. The reaction rates were fitted to the Michaelis–Menten equation to calculate the kinetic constants: v = Vmax [S]/(Km + [S]), where v is the initial velocity, Vmax is the maximal reaction velocity, [S] is the concentration of substrate and Km is the Michaelis constant. Kcat is the catalytic constant, where Kcat = Vmax/[E]. [E] is the molar concentration of Co3O4 nanomaterials (or horseradish peroxidase). A Co3O4 nanoparticle was set as an enzyme molecule.3 The molar concentrations of Co3O4 nanomaterials were calculated using the lattice parameter of Co3O4, the volume of a nanoparticle and the mass concentrations. Prior to the surface modification, the graphite electrodes (GE, 3.0 mm in diameter) were firstly polished with 0.3 and 0.05 mm alumina slurry, and then cleaned ultrasonically in
This journal is © the Owner Societies 2014
View Article Online
Published on 30 May 2014. Downloaded by University of Tasmania on 31/08/2014 14:55:42.
PCCP
Paper
distilled water and ethanol successively. The three Co3O4 nanomaterials were dispersed into distilled water by ultrasonic dispersion to obtain the three suspensions of Co3O4 nanomaterials (3.09 mg mL1 nanoplates, 2.91 mg mL1 nanorods or 4.07 mg mL1 nanocubes). And three colloidal solutions (5 mL) were then dropped on three pretreated GE surfaces and allowed to dry at 70 1C. Then, an aliquot of 2 mL of nafion solution (0.5 wt%) was cast on the layer of Co3O4 nanomaterials and dried at 70 1C. Cyclic voltammetric and amperometric measurements were performed on a CHI 660D (Chenhua, China). A three-electrode system comprising a platinum plate as auxiliary, a saturated calomel electrode (SCE) as reference and the Co3O4 nanomaterials-modified electrode as working electrodes was used for all electrochemical experiments.
3. Results and discussion 3.1. Characterization of Co3O4 nanomaterials with different shapes The morphology of the synthesized products was characterized by SEM and TEM. Fig. 1A and B show that the Co3O4 nanoplates are uniform and their average edge length and thickness are 100 nm and 21 nm, respectively. Fig. 1C and D show that the Co3O4 nanorods have a diameter of 10 nm and the length ranging from 40 nm to 80 nm. And Co3O4 nanocubes are monodispersed with a uniform edge of 20 nm (Fig. 1E and F). The crystallographic nature of the three Co3O4 nanomaterials was examined using high-resolution TEM (HRTEM) measurements.
Fig. 1 SEM images of Co3O4 nanoplates (A), Co3O4 nanorods (C) and Co3O4 nanocubes (E), TEM images of Co3O4 nanoplates (B), Co3O4 nanorods (D) and Co3O4 nanocubes (F).
This journal is © the Owner Societies 2014
Fig. 2 (A) TEM image of a typical Co3O4 nanoplate, HRTEM images of site 1 (B), site 2 (C) and site 3 (D) of Co3O4 nanoplate, (E) a HRTEM image of a Co3O4 nanorod, (F) a HRTEM image of a Co3O4 nanocube, and (G) exposed crystal planes on the nanoplate, nanorod, and nanocube of Co3O4.
The HRTEM images of selected sites 1, 2, and 3 of a typical Co3O4 nanoplate are shown in Fig. 2A–D. The dominant exposed planes of Co3O4 nanoplates are {112} planes, which are the only planes normal to both the set of (220) planes with a lattice spacing of 0.28 nm and the set of (222) planes with a square crossing lattice spacing of 0.23 nm.26 As is shown in Fig. 2E, both the set of (311) planes with a lattice spacing of 0.24 nm and set of (111) planes with a crossing lattice spacing of 0.467 nm are observed for Co3O4 nanorods. Therefore, the predominantly exposed planes of Co3O4 nanorods are {110} planes.37 The HRTEM image of Co3O4 nanocubes is shown in Fig. 2F. The dominant exposed planes of Co3O4 nanocubes are {100} planes, which are the only planes normal to the set of (400) planes with a lattice spacing of 0.20 nm.38 The crystal structures of the three Co3O4 nanomaterials were determined by X-ray powder diffraction (XRD). Despite variations in shape, all the patterns obtained can be indexed to the spinel structure of Co3O4 in good agreement with the reported data (JCPDS no. 76-1802) and no impurity peaks are observed, indicating the high purity of the final products (Fig. 3).39 The three Co3O4 nanomaterials were synthesized in the absence of organic macromolecular surfactants. The IR spectra of the three Co3O4 nanomaterials all display two sharp bands at 578 cm1 (n1) and 666 cm1 (n2), a broad band at 3409 cm1, and no other evident bands (Fig. S1, ESI†). The two sharp bands originate from the stretching vibrations of the metal–oxygen bond.40 The n1 band at 578 cm1 is characteristic of Co3+ vibration in the octahedral hole, and the n2 band at 666 cm1 is attributable to Co2+ vibration in the tetrahedral hole in the spinel lattice, confirming the formation of Co3O4 spinel oxide. The broad band at 3409 cm1 is assigned to the O–H stretching vibration due to the adsorbed H2O molecules.8 As a result, the products are free of coatings surrounding their surfaces. Therefore, the effect of coatings on their catalytic ability is excluded.
Phys. Chem. Chem. Phys., 2014, 16, 15709--15716 | 15711
View Article Online
Published on 30 May 2014. Downloaded by University of Tasmania on 31/08/2014 14:55:42.
Paper
PCCP
Fig. 3 XRD patterns of (a) Co3O4 nanoplates, (b) Co3O4 nanorods and (c) Co3O4 nanocubes.
The N2 adsorption–desorption isotherms of the three Co3O4 nanomaterials are shown in Fig. S2 (ESI†). The specific surface areas of Co3O4 nanoplates, nanorods and nanocubes are 97.1, 103.2 and 73.7 m2 g1, respectively. 3.2. The peroxidase-like catalytic activities of different Co3O4 nanomaterials The natural peroxidases can catalyze the oxidation of the substrates (amines and phenols) in the presence of H2O2 to develop the colour change.41 Therefore, the catalysis of TMB by the Co3O4 nanomaterials was tested in the presence of H2O2. In order to exclude the factor of surface area affecting the catalytic properties, the amount of each Co3O4 nanomaterials provides the same surface area in the reaction system.42 As shown in Fig. 4, in the absence of any Co3O4 nanomaterials, the solution containing TMB and H2O2 is colourless and has no absorption in the range from 500 to 800 nm. However, in the presence of any Co3O4 nanomaterials, the solutions produce a blue colour with absorbance maxima at 652 nm, but the absorbance intensity varies with the different shapes of
Fig. 4 The relative peroxidase-like activity of Co3O4 nanomaterials with TMB as the substrate. (1) Nanoplates; (2) nanorods; (3) nanocubes; and (4) in the absence of Co3O4 nanomaterials. The inset show the timedependent catalytic activity of Co3O4 nanomaterials with different shapes. The photograph showed the colour development of different samples after 5 min. The amount of each Co3O4 nanomaterials corresponded to a surface area of 3 103 m2. Experiments were carried out using 10.3 mg mL1 Co3O4 nanoplates (9.7 mg mL1 Co3O4 nanorods or 13.6 mg mL1 Co3O4 nanocubes), 0.5 mM TMB and 50 mM H2O2 in 3 mL phosphate buffer (100 mM, pH 6) and measured after 5 min using a UV-vis spectrometer. The time-dependent catalyses were measured at 652 nm.
15712 | Phys. Chem. Chem. Phys., 2014, 16, 15709--15716
Fig. 5 The relative peroxidase-like activity of Co3O4 nanomaterials with DAB (A) and OPD (B) as substrates. The insets show the time-dependent catalytic activity of Co3O4 nanomaterials with different shapes. The photographs showed the colour development of different samples after 15 min. (1) nanoplates; (2) nanorods; (3) nanocubes; and (4) in the absence of Co3O4 nanomaterials. Experiments were carried out using 10.3 mg mL1 Co3O4 nanoplates (9.7 mg mL1 Co3O4 nanorods or 13.6 mg mL1 Co3O4 nanocubes), 1 mM DAB and 200 mM H2O2 in 100 mM phosphate buffer (pH 4), or 10 mM OPD and 100 mM H2O2 in 100 mM phosphate buffer (pH 6), and then measured after 15 min using a UV-vis spectrometer. The timedependent catalyses were measured at 464 nm (DAB) and 436 nm (OPD).
Co3O4. The absorption originates from the oxidation of TMB, similar to the phenomenon observed for horseradish peroxidase (HRP).3 And the difference in absorbance intensity is attributed to the different catalytic activities of Co3O4 nanomaterials. The inset of Fig. 4 also shows the time-dependent catalytic activity of different Co3O4 nanomaterials under the standard reaction conditions. The different Co3O4 nanomaterials exhibit different levels of activity over the reaction time, following the order of nanoplates c nanorods 4 nanocubes. To further characterize the different peroxidase-like catalytic activities of the Co3O4 nanomaterials, the experiments were conducted using other usually used peroxidase substrates in place of TMB, including DAB and OPD. As shown in Fig. 5A, with any of the Co3O4 nanomaterials, the solutions of DAB and H2O2 display a brown colour, which originates from the oxidation product of DAB, but there is still a difference in absorbance intensity for different Co3O4 nanomaterials. A similar catalytic performance is observed when OPD is used as the substrate (Fig. 5B). The three results all indicate that different Co3O4 nanomaterials own different levels of catalytic activity, in the order of nanoplates c nanorods 4 nanocubes. To exclude the difference in their catalytic activity resulting from the different leaching of cobalt ions, the Co3O4 nanomaterials were incubated in reaction buffer (pH 6.0) for 30 min
This journal is © the Owner Societies 2014
View Article Online
PCCP
Paper
and then removed by centrifugation. The catalytic activities of the leaching solutions were tested under standard conditions. All of the leaching solutions have a negligible absorption (data not shown), indicating that their catalytic activities are derived from the intact Co3O4 nanomaterials themselves rather than as a result of leaching cobalt ions.
Published on 30 May 2014. Downloaded by University of Tasmania on 31/08/2014 14:55:42.
3.3. Effect of external conditions on the catalytic activity of the three Co3O4 nanomaterials The substrates TMB and H2O2 are selected as the model systems to investigate the catalytic properties of Co3O4 nanomaterials with different shapes. In order to understand the effect of external conditions on the catalytic ability of the Co3O4 nanomaterials, their catalytic activities were measured by varying the pH from 2 to 8 and the temperature from 20 to 50 1C. Fig. 6 shows that the optimal pH is 6, indicating that the oxidation of TMB occurs easily under weakly acidic conditions, as have also been reported for HRP. And the optimal temperature is 25 1C, exhibiting the high catalytic ability even at room temperature. All the Co3O4 nanomaterials as peroxidase mimics have the same optimal pH and temperature, thus, pH 6 and 25 1C are adopted as standard reaction conditions for the subsequent study of their catalytic properties. The above results indicate that the different catalytic activities of Co3O4 nanomaterials are not associated with the external conditions but their different shapes. 3.4.
The kinetic study of Co3O4 nanomaterials
kinetic data were measured by varying the concentration of one substrate while keeping the concentration of the other substrate constant. At a fixed concentration of one substrate, typical Michaelis–Menten curves are observed for Co3O4 nanoplates, Co3O4 nanorods and Co3O4 nanocubes (Fig. 7). The Lineweaver–Burk double reciprocal plots (Fig. S3, ESI†) show good linear relationship between v1 and [S]1. The Vmax and Km of the three Co3O4 nanomaterials could be obtained by the slopes and intercepts of these lines. The experimental data are well fitted to the Michaelis–Menten equation to obtain the parameters shown in Table 1. These results indicate that the reaction catalyzed by the three Co3O4 nanomaterials all follow the typical Michaelis–Menten model. The Vmax values with H2O2 and TMB as the two substrates all increase from nanocubes through nanorods to nanoplates, showing the highest maximal reaction velocity. On the other hand, the lowest Km values with the two substrates are shown with the nanoplates followed by the nanorods and then the nanocubes. Vmax values are the indicators of reaction activity (i.e. the rate of reaction when an enzyme is saturated with the substrate). The Vmax values of the three Co3O4 nanomaterials show that nanoplates have the highest peroxidase-like catalytic activity and nanorods exhibit medium activity, higher than nanocubes, which is consistent with the above results. Km values represent the affinity of the enzyme towards the substrate. The lower the Km values, the greater the affinity. The Km values of the three Co3O4 nanomaterials suggest that nanoplates have the highest affinity, nanorods ranked the second place, the Km values of
The peroxidase-like catalytic properties of the Co3O4 nanomaterials were further investigated using steady-state kinetics. Co3O4 nanomaterials catalyzed the bisubstrate reaction, so the
Fig. 6 Effects of pH (A) and temperature (B) on the catalytic activity of Co3O4 nanoplates (’), Co3O4 nanorods (K) and Co3O4 nanocubes (m). Experiments were carried out using 10.3 mg mL1 Co3O4 nanoplates (9.7 mg mL1 Co3O4 nanorods or 13.6 mg mL1 Co3O4 nanocubes), 0.5 mM TMB and 50 mM H2O2. (A) 100 mM Na2HPO4-acetic buffers of different pHs. (B) 100 mM phosphate buffer (pH 6). The maximum point in each curve was set to 100%.
This journal is © the Owner Societies 2014
Fig. 7 Steady-state kinetic assay. The velocity of the reaction was measured using 10.3 mg mL1 Co3O4 nanoplates (9.7 mg mL1 Co3O4 nanorods or 13.6 mg mL1 Co3O4 nanocubes) in 100 mM phosphate buffer (pH 6.0). (A), (C) and (E) The concentration of H2O2 was 50 mM and the TMB concentration was varied. (B), (D) and (F) The concentration of TMB was 0.5 mM and the H2O2 concentration was varied.
Phys. Chem. Chem. Phys., 2014, 16, 15709--15716 | 15713
View Article Online
Paper
Published on 30 May 2014. Downloaded by University of Tasmania on 31/08/2014 14:55:42.
Table 1
PCCP Comparison of morphological features and kinetic parameters of various Co3O4 nanomaterials and HRP
Surface area/m2 g1
Substrates
Km/mM
Vmax/108 M s1
Kcat/103 s1
0.90
TMB H2O2
0.090 284
9.91 48.08
11.01 53.42
103.2
44.29
TMB H2O2
0.22 455
7.12 39.60
0.16 0.89
73.7
46.50
TMB H2O2
0.26 480
6.04 35.81
0.13 0.77
2.50
TMB H2O2
0.43 3.70
10.00 8.71
4.00 3.48
Catalyst
Exposed planes
Size/nm
Nanoplates
{112}
21 (thickness) 100 (edge)
97.1
Nanorods
{110}
10 (diameter) 40–80 (length)
Nanocubes
{100}
20 (edge)
HRPa a
[E]/1011 M
Obtained by the previous report.3
both are higher than that of nanocubes. This may be due to the presence of more ‘‘active sites’’ on the surface of the Co3O4 nanoplates compared to nanorods and nanocubes, and the substrates are able to coordinate more easily with the metal centers.8 Therefore, the Co3O4 nanoplates exhibit the highest affinity and catalytic activity. In order to compare the catalytic activities of Co3O4 nanomaterials and natural peroxidase, their Kcat values (catalytic constants) were obtained. The Kcat values of Co3O4 nanorods and nanocubes are smaller than those of HRP, indicating that the catalytic activities of Co3O4 nanorods and nanocubes are lower than that of HRP. However, the increase of catalytic activities from HRP to Co3O4 nanoplates is by one order of magnitude based on their Kcat values. The results demonstrate that shape-controllable Co3O4 nanomaterials can be highly efficient peroxidase mimics. 3.5. Electrocatalytic activity of Co3O4 nanomaterials toward reduction of H2O2 The direct electrochemistry of heme-containing enzymes at electrodes has been well studied in order to understand the electron-transfer process of enzymes.43 It has been shown that the catalytic activity of some nanoparticles as enzyme mimics originated from their electron transfer ability.4,8 Therefore, the electrocatalytic behaviour of the Co3O4 nanomaterials modified electrode towards the reduction of H2O2 was studied using cyclic voltammetry and amperometric responses. In the presence of H2O2, no obvious current is found with bare GE, but evident peaks of reduction at 0.55 V are observed using the Co3O4 nanomaterials modified GE (Fig. 8A). The reduction currents at 0.55 V increase steeply to reach a steady-state value with addition of an aliquot of H2O2 (Fig. 8B). The three Co3O4 nanomaterials modified GE show different reduction currents, following the order of nanoplates c nanorods 4 nanocubes. The results demonstrate clearly that three Co3O4 nanomaterials exhibit different electrocatalytic activities towards the reduction of H2O2, which indicate that they have the different abilities of electron transfer between the electrode (electron donor) and H2O2 (electron acceptor). The {100} plane contains only one Co2+ cation which is not favourable for its electron transfer (Fig. 9). There are both Co2+ cations and Co3+ cations on {110} and {112} planes (Fig. 9), so their electron transfer abilities are higher than that of the {100} plane. The {112} plane exhibits the
15714 | Phys. Chem. Chem. Phys., 2014, 16, 15709--15716
Fig. 8 (A) Cyclic voltammograms of the Co3O4 nanomaterials modified GE electrodes, 40 mM H2O2. (B) Amperometric response of the Co3O4 nanomaterials modified GE at the applied potential of 0.55 V upon successive addition of 2.5 mM H2O2. Nanoplates (1), nanorods (2), nanocubes modified GE (3) and bare GE (4), 100 mM phosphate buffer (pH 6).
highest electron transfer, which arises from its particular atom arrangement. In one word, the three crystal planes have different arrangements of surface atoms, thus exhibiting different electron transfer abilities, which induce their different peroxidaselike catalytic activities. 3.6. Crystal plane effect on the catalytic activities of Co3O4 nanomaterials The above results showed that the three kinds of Co3O4 nanomaterials exhibit obviously different peroxidase-like catalytic activities. Co3O4 nanoplates with {112} planes exhibit not only the highest catalytic rate but also the best affinity towards the substrates. The catalytic activity of the three Co3O4 nanomaterials is ranked in the order ‘‘nanoplates ({112} planes) c nanorods ({110} planes) 4 nanocubes ({100} planes)’’, which could be largely attributed to the ‘‘crystal plane’’ effect. Regarding the same surface areas of the three kinds of Co3O4
This journal is © the Owner Societies 2014
View Article Online
Published on 30 May 2014. Downloaded by University of Tasmania on 31/08/2014 14:55:42.
PCCP
Fig. 9 The surface atomic configurations in the {112}, {110} and {100} planes of the Co3O4 crystal. The Co3O4 Crystallographic Information File (CIF) was taken from the NIST/FIZ Inorganic Crystal Structure Database. The surface atomic configurations were drawn by the CIF and the software of Materials Studio.26
nanomaterials in the reaction systems, this further confirms that the differences in their peroxidase-like catalytic activity originate from the crystal plane effect and the well-defined crystal plane is the dominant factor in Co3O4 nanomaterials. The surface atomic configurations in the {112}, {110} and {100} planes of the Co3O4 unit cell are shown in Fig. 9. According to the surface atomic configuration in a unit cell, the surface layer of the {100} plane contains one Co2+ cation on average and does not contain Co3+ cations, but there are two Co3+ cations in the sub-layer. It has been explained that the catalytic activity of metal oxide NPs originated from the ion pairs (catalytic sites in two different oxidation states).8,44 So it is difficult for the substrates to come into contact with the Co3+ cations during the catalyzed cycle by Co3O4 nanocubes. The Co3+ cations on the sub-layers are exposed possibly in the defects, and these defects are usually the active sites, which contribute considerably to the catalysis. Therefore, Co3O4 nanocubes with {100} planes exhibit lower catalytic activity than the other two Co3O4 nanomaterials. As illustrated in the atomic configurations (Fig. 9), unlike Co3O4 nanocubes with {100} planes, there are not only Co2+ cations but also Co3+ cations in Co3O4 nanorods with {110} planes and Co3O4 nanoplates with {112} planes. Although both the {110} and {112} planes contain two Co2+ cations and two Co3+ cations, respectively, their arrangement manner of surface atoms is different (as shown in Fig. 9). The high-index plane of {112} possesses a more open structure than that of the {110} plane, and its density of stepped atoms is higher than that of the {110} plane (Fig. 9). Therefore, the {112} plane has a more reactive surface which leads to the highest catalytic ability in the three planes.
4. Conclusions Co3O4 nanomaterials with different shapes including nanoplates, nanorods and nanocubes, were synthesized. The different
This journal is © the Owner Societies 2014
Paper
Co3O4 nanomaterials expose different crystal planes, and the predominantly exposed planes of nanoplates, nanorods and nanocubes are {112}, {110} and {100} planes, respectively. They exhibit different peroxidase-like catalytic activities, in the order of nanoplates c nanorods 4 nanocubes. The catalytic performances all follow the typical Michaelis–Menten model. The Vmax values show that nanoplates have the highest peroxidase-like catalytic activity and nanorods exhibit medium activity, higher than nanocubes. The Km values suggest that nanoplates have the highest affinity towards the substrate (TMB and H2O2), nanorods ranked the second place, the Km values of both are higher than that of nanocubes. Compared with natural peroxidase, the catalytic activity of Co3O4 nanoplates is even higher than that of HRP. The three Co3O4 nanomaterials as peroxidase mimics have the same optimal pH and temperature, indicating that their different catalytic activities are not associated with the external conditions but their different crystal planes. Therefore, the crystal plane of Co3O4 has a significant influence on their catalytic activities. The catalytic activity of the three crystal planes is ranked as: {112} c {110} 4 {100}. Their different catalytic activities depend greatly upon the different arrangements of surface atoms. The three crystal planes exhibit different electron transfer abilities, which induce the different peroxidaselike catalytic activities of Co3O4 nanomaterials. This study shows that the crystal planes obtained by shape control of Co3O4 nanomaterials can regulate the peroxidase-like catalytic activity. This suggests that we can design and synthesize the nanomaterials with well-defined crystal planes to improve their catalytic activity. The concept of crystal plane-dependent nanocatalysis not only enables the fundamental understanding of the catalytic phenomenon but also highlights the essential implications for the design and preparation of more efficient catalysts.
Acknowledgements The authors acknowledge the financial support from National Natural Science Foundation of China under the project number 21273057.
Notes and references 1 R. Breslow, Acc. Chem. Res., 1995, 28, 146–153. 2 Y. Murakami, J. Kikuchi, Y. Hisaeda and O. Hayashida, Chem. Rev., 1996, 96, 721–758. 3 L. Gao, J. Zhuang, L. Nie, J. Zhang, Y. Zhang, N. Gu, T. Wang, J. Feng, D. Yang, S. Perrett and X. Yan, Nat. Nanotechnol., 2007, 2, 577–583. 4 Y. Song, K. Qu, C. Zhao, J. Ren and X. Qu, Adv. Mater., 2010, 22, 2206–2210. 5 W. He, X. Wu, J. Liu, X. Hu, K. Zhang, S. Hou, W. Zhou and S. Xie, Chem. Mater., 2010, 22, 2988–2994. ´, F. Nata ´lio, M. Humanes, J. Leppin, K. Heinze, 6 R. Andre ¨der, W. E. Mu ¨ller and W. Tremel, R. Wever, H. C. Schro Adv. Funct. Mater., 2011, 21, 501–509.
Phys. Chem. Chem. Phys., 2014, 16, 15709--15716 | 15715
View Article Online
Published on 30 May 2014. Downloaded by University of Tasmania on 31/08/2014 14:55:42.
Paper
7 H. Deng, W. Shen, Y. Peng, X. Chen, G. Yi and Z. Gao, Chem. – Eur. J., 2012, 18, 8906–8911. 8 J. Mu, Y. Wang, M. Zhao and L. Zhang, Chem. Commun., 2012, 48, 2540–2542. 9 J. Mu, L. Zhang, M. Zhao and Y. Wang, J. Mol. Catal. A: Chem., 2013, 378, 30–37. 10 E. G. Heckert, A. S. Karakoti, S. Seal and W. T. Self, Biomaterials, 2008, 29, 2705–2709. 11 T. Pirmohamed, J. M. Dowding, S. Singh, B. Wasserman, E. Heckert, A. S. Karakoti, J. E. S. King, S. Seal and W. T. Self, Chem. Commun., 2010, 46, 2736–2738. 12 C.-W. Tseng, H.-Y. Chang, J.-Y. Chang and C.-C. Huang, Nanoscale, 2012, 4, 6823–6830. 13 W. Chen, J. Chen, A. L. Liu, L. M. Wang, G. W. Li and X. H. Lin, ChemCatChem, 2011, 3, 1151–1154. 14 L. Gao, J. Wu, S. Lyle, K. Zehr, L. Cao and D. Gao, J. Phys. Chem. C, 2008, 112, 17357–17361. ¨gl and S. B. Abd Hamid, Angew. Chem., Int. Ed., 15 R. Schlo 2004, 43, 1628–1637. 16 M. Valden, X. Lai and D. W. Goodman, Science, 1998, 281, 1647–1650. ¨ckner, S. Zhang and 17 F. Shi, M. K. Tse, M. M. Pohl, A. Bru M. Beller, Angew. Chem., Int. Ed., 2007, 46, 8866–8868. 18 N. Spencer, R. Schoonmaker and G. Somorjai, Nature, 1981, 294, 643–644. 19 N. Spencer, R. Schoonmaker and G. Somorjai, J. Catal., 1982, 74, 129–135. 20 J. A. van Bokhoven, ChemCatChem, 2009, 1, 363–364. 21 B. Lim, M. Jiang, J. Tao, P. H. Camargo, Y. Zhu and Y. Xia, Adv. Funct. Mater., 2009, 19, 189–200. 22 B. Lim, J. Wang, P. H. Camargo, C. M. Cobley, M. J. Kim and Y. Xia, Angew. Chem., Int. Ed., 2009, 48, 6304–6308. 23 Y. w. Jun, J. s. Choi and J. Cheon, Angew. Chem., Int. Ed., 2006, 45, 3414–3439. 24 S. Agarwal, L. Lefferts, B. L. Mojet, D. Ligthart, E. J. Hensen, D. R. Mitchell, W. J. Erasmus, B. G. Anderson, E. J. Olivier and J. H. Neethling, ChemSusChem, 2013, 6, 1898–1906. 25 Z. A. Qiao, Z. Wu and S. Dai, ChemSusChem, 2013, 6, 1821–1833.
15716 | Phys. Chem. Chem. Phys., 2014, 16, 15709--15716
PCCP
26 L. Hu, Q. Peng and Y. Li, J. Am. Chem. Soc., 2008, 130, 16136–16137. 27 X. Xie, Y. Li, Z.-Q. Liu, M. Haruta and W. Shen, Nature, 2009, 458, 746–749. 28 E. Aneggi, D. Wiater, C. de Leitenburg, J. Llorca and A. Trovarelli, ACS Catal., 2014, 4, 172–181. 29 I. Lee, F. Delbecq, R. Morales, M. A. Albiter and F. Zaera, Nat. Mater., 2009, 8, 132–138. 30 N. Tian, Z.-Y. Zhou, S.-G. Sun, Y. Ding and Z. L. Wang, Science, 2007, 316, 732–735. 31 L. Xiao, L. Zhuang, Y. Liu and J. Lu, J. Am. Chem. Soc., 2008, 131, 602–608. 32 G. Centi and S. Perathoner, Eur. J. Inorg. Chem., 2009, 3851–3878. 33 J. Zeng, Q. Zhang, J. Chen and Y. Xia, Nano Lett., 2009, 10, 30–35. 34 D.-e. Jiang and S. Dai, Phys. Chem. Chem. Phys., 2011, 13, 978–984. 35 S. Hwang, A. Umar, S. Kim, S. Al-Sayari, M. Abaker, A. Al-Hajry and A. M. Stephan, Electrochim. Acta, 2011, 56, 8534–8538. 36 X. C. Song, X. Wang, Y. F. Zheng, R. Ma and H. Y. Yin, J. Nanopart. Res., 2011, 13, 1319–1324. 37 Z. Fei, S. He, L. Li, W. Ji and C.-T. Au, Chem. Commun., 2012, 48, 853–855. 38 R. Xu and H. C. Zeng, Langmuir, 2004, 20, 9780–9790. 39 B. Varghese, C. Teo, Y. Zhu, M. V. Reddy, B. V. Chowdari, A. T. S. Wee, V. Tan, C. T. Lim and C. H. Sow, Adv. Funct. Mater., 2007, 17, 1932–1939. 40 J. Yin, H. Cao and Y. Lu, J. Mater. Chem., 2012, 22, 527–534. 41 A. M. Klibanov, T.-m. Tu and K. P. Scott, Science, 1983, 221, 259–261. 42 Y. Teng, Y. Kusano, M. Azuma, M. Haruta and Y. Shimakawa, Catal. Sci. Technol., 2011, 1, 920–922. 43 L. Gorton, A. Lindgren, T. Larsson, F. Munteanu, T. Ruzgas and I. Gazaryan, Anal. Chim. Acta, 1999, 400, 91–108. 44 I. E. Wachs, Catal. Today, 2005, 100, 79–94.
This journal is © the Owner Societies 2014
Published on 30 May 2014. Downloaded by University of Tasmania on 31/08/2014 14:55:42.
PAPER
Cite this: Phys. Chem. Chem. Phys., 2014, 16, 15709
View Journal | View Issue
The crystal plane effect on the peroxidase-like catalytic properties of Co3O4 nanomaterials† Jianshuai Mu, Li Zhang, Guangyu Zhao and Yan Wang* Nanomaterials as enzyme mimics have received considerable attention as they can overcome some serious disadvantages associated with the natural enzymes. In recently developed Co3O4 nanoparticles as peroxidase mimics, the influence of the crystal plane on the catalytic performance has not been demonstrated. In order to better understand their crystal plane-dependent catalysis, the present study was initiated using three different Co3O4 nanomaterials, nanoplates, nanorods and nanocubes, as model systems. According to HRTEM, the predominantly exposed planes of nanoplates, nanorods and nanocubes are {112}, {110} and {100} planes, respectively. The catalytic activities were explored by using H2O2 and different organic substrates as the substrates of peroxidase mimics, and were investigated in-depth by steady-state kinetics and electrochemistry methods in depth. The results show that the peroxidase-like activity increases from nanocubes to nanoplates, via nanorods. The effect of external conditions such as pH and temperature on the three nanomaterials is the same, which indicates that the difference in their catalytic activities originates from their different shapes. The peroxidase-like catalytic activities of Co3O4 nanomaterials are crystal plane-dependent and follow the order: {112} c {110} 4 {100}. The three crystal
Received 27th March 2014, Accepted 27th May 2014
planes have different arrangements of surface atoms, thus exhibiting different abilities of electron transfer,
DOI: 10.1039/c4cp01326c
which induce their different peroxidase-like catalytic activities. This investigation clarifies that the
www.rsc.org/pccp
that Co3O4 nanomaterials can serve as catalyst models for designing other catalysts.
peroxidase-like activity of Co3O4 nanomaterials can be enhanced by shape control. These findings show
1. Introduction Artificial enzymes are a rapidly developing field due to the intrinsic drawbacks associated with the natural enzymes, such as low stability, undesirable sensitivity of catalytic activity to environmental conditions, requirement of special storage conditions and high cost.1,2 Several years ago, Fe3O4 nanoparticles were discovered to have the peroxidase-like activity and could be used as peroxidase mimics to catalyze and detect some molecules.3 This gave rise to new possibilities in the field of nanomaterial-based artificial enzymes. With the developments in the field, other nanomaterials have also been evaluated as enzyme mimics.4–13 The nanomaterialbased enzyme mimics take advantage of low cost, high stability and tunability of catalytic activities and can be used in bioassays and medical diagnostics.14 Nanocatalysis is an explosively growing topic because of its considerable contributions to both materials science and heterogeneous catalysis. Researchers have long been seeking catalysts with small nanometer-sized particles, because the
Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin 150001, China. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4cp01326c
This journal is © the Owner Societies 2014
nanoscale catalysts should offer higher surface areas, lowcoordinated sites, and surface vacancies, which are responsible for the higher catalytic activity.15–17 It has been shown that the catalytic performance of a catalyst is associated with not only the size but also the shape. By varying the shape of the crystals, the exposed crystal planes are affected, thus influencing the catalytic performance. For instance, the orders of magnitude variations in the rate of ammonia synthesis were reported on different planes of iron surfaces (the contribution recognized with a Nobel Prize in 2007).18,19 The planes of technical catalysts are poorly defined which may lead to nonideal catalytic performance. It also hampers the understanding of the catalytic nature, which is one of the major challenges in heterogeneous catalysis. Therefore shapecontrolled nanocatalysts with well-defined planes are novel simple systems in order to facilitate the design and fabrication of highly efficient catalysts.20 During the past decade, the rapid development of material science has offered an opportunity to prepare metal and/ or metal oxide nanomaterials with specific planes exposed.21–23 It is now time to discuss whether there are new ideas regarding shape that can lead catalysis research into new directions. Several recent studies have demonstrated the importance of a well-defined shape/ plane in enhancing catalytic performance.24–34 Therefore, shapecontrollable synthesis of nanocatalysts is an excellent way of obtaining the next generation high-performance catalysts.
Phys. Chem. Chem. Phys., 2014, 16, 15709--15716 | 15709
View Article Online
Published on 30 May 2014. Downloaded by University of Tasmania on 31/08/2014 14:55:42.
Paper
PCCP
Not long ago, we firstly found that the Co3O4 nanoparticles exhibited the peroxidase-like catalytic activity.8 However, their peroxidase-like catalytic properties with different crystal planes have not been addressed. In order to understand this crystal plane effect, the Co3O4 nanomaterials with different shapes were chosen as model systems. Their different shapes exposed different crystal planes. The peroxidase-like catalytic performance of Co3O4 nanomaterials was studied using typical substrates, such as 3,3 0 ,5,5 0 -tetramethylbenzidine (TMB), o-phenylenediamine (OPD) and 3,3 0 -diaminobenzidine (DAB), in the presence of H2O2, and the steady-state kinetics of the three nanomaterials were also investigated. Then, a correlative investigation between the crystal planes of Co3O4 nanomaterials and their catalytic performance is presented. This work not only favours the deep understanding of the catalytic phenomenon, but also provides a potential way to develop more efficient catalysts for some applications.
2. Experimental section 2.1.
Materials
All chemicals used for the synthesis of Co3O4 nanomaterials were of analytical grade and used as received without further purification. CoCl26H2O, ammonium hydroxide, Co(NO3)26H2O, hexamethylenetetramine, sodium hydroxide, Co(CH3COO)24H2O, sodium carbonate, ethylene glycol and hydrogen peroxide were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). The peroxidase substrates including 3,30 ,5,50 -tetramethylbenzidine (TMB), o-phenylenediamine (OPD) and 3,3 0 -diaminobenzidine (DAB) were obtained from Sigma-Aldrich (St. Louis, USA). 2.2.
Preparation of Co3O4 nanomaterials
Co3O4 nanoplates were synthesized by a hydrothermal method according to the reported route.35 In a typical reaction process, for the synthesis of Co3O4 nanoplates, 1.3 g of Co(NO3)26H2O was dissolved in 100 mL of water, and mixed with 100 mL solution containing 0.6 g of hexamethylenetetramine (HMTA), under continuous stirring. A few drops of NaOH solution were then added into the above solution to obtain pH 10. After maintaining the pH, the solution was vigorously stirred for 2 h. Consequently, the solution was then loaded into the Teflonlined stainless steel autoclaves, sealed and heated up to 110 1C for 15 h. After completing the reaction, the autoclave was naturally allowed to cool at room-temperature and the products were washed with water, ethanol and acetone sequentially and dried at room temperature. The dried products were then calcined at 200 1C for 2 h. The Co3O4 nanorods were prepared according to the method reported in the literature.27 3.32 g of Co(CH3COO)24H2O was dissolved in 40 mL of ethylene glycol and the mixture was gradually heated to 160 1C. Under a continuous flow of nitrogen, 134 mL of aqueous 0.2 M sodium carbonate solution was then added at the rate of 1.1 mL min1 and the slurry was further aged for 1 h under vigorous stirring. After centrifugation and washing with water, the solid obtained was dried at
15710 | Phys. Chem. Chem. Phys., 2014, 16, 15709--15716
50 1C overnight under vacuum and then calcined at 450 1C for 4 h in air. The Co3O4 nanocubes were synthesized according to the literature.36 1 g of Co(NO3)26H2O was dissolved in 40 mL of water and mixed with 0.8 mL of 30 wt% hydrogen peroxide. And pH of the solution was maintained at 9 by adding ammonia. Then the reaction mixture was charged in a 50 mL capacity autoclave with Teflon liner followed by uniform heating at 180 1C for 12 h. After the reaction was completed, the autoclave was allowed to cool to room temperature naturally. The solid precipitate was collected by centrifugation, washed several times with water, and then dried at 80 1C in air. 2.3. Characterization of differently shaped Co3O4 nanomaterials The composition and phase of the synthesized product were identified by powder X-ray diffraction (XRD) on an X 0 Pert PRO X-ray diffractometer (PANalytical, Netherlands) using Cu Ka radiation (l = 1.5418 Å). The morphology and size of the product were examined by scanning electron microscopy (SEM) using a SU-8000 SEM (Hitachi, Japan) at an accelerating voltage of 15 kV. The structure and composition of the products were characterized by means of a high-resolution transmission electron microscope (HRTEM, Tecnai G2 F30, USA). FT-IR spectra were recorded on a Spectrum 100 Frontier IR Spectrometer (Perkin Elmer, USA). The N2 adsorption–desorption isotherms of the Co3O4 nanomaterials were measured on an ASAP 2020 Physisorption Analyzer (Micromeritics, USA) at 77 K, and the specific surface areas were obtained by the Brunauer– Emmett–Teller (BET) method. 2.4. Catalytic experiments and electrochemical measurements The peroxidase-like catalytic activities of the Co3O4 nanomaterials were examined in phosphate buffer (3 mL, 100 mM, pH 6.0) containing Co3O4 nanomaterials, in the presence of H2O2 and the organic substrate (TMB, OPD or DAB). The reactions were monitored in wavelength-scan mode or time-drive mode by using a Lambda 750 UV-vis-NIR spectrophotometer (Perkin Elmer, USA). The apparent steady-state reaction rates of three Co3O4 nanomaterials were deduced according to the initial linear range of the kinetic curves and the molar absorption coefficient of 39 000 M1 cm1 for TMB-derived oxidation products. The reaction rates were fitted to the Michaelis–Menten equation to calculate the kinetic constants: v = Vmax [S]/(Km + [S]), where v is the initial velocity, Vmax is the maximal reaction velocity, [S] is the concentration of substrate and Km is the Michaelis constant. Kcat is the catalytic constant, where Kcat = Vmax/[E]. [E] is the molar concentration of Co3O4 nanomaterials (or horseradish peroxidase). A Co3O4 nanoparticle was set as an enzyme molecule.3 The molar concentrations of Co3O4 nanomaterials were calculated using the lattice parameter of Co3O4, the volume of a nanoparticle and the mass concentrations. Prior to the surface modification, the graphite electrodes (GE, 3.0 mm in diameter) were firstly polished with 0.3 and 0.05 mm alumina slurry, and then cleaned ultrasonically in
This journal is © the Owner Societies 2014
View Article Online
Published on 30 May 2014. Downloaded by University of Tasmania on 31/08/2014 14:55:42.
PCCP
Paper
distilled water and ethanol successively. The three Co3O4 nanomaterials were dispersed into distilled water by ultrasonic dispersion to obtain the three suspensions of Co3O4 nanomaterials (3.09 mg mL1 nanoplates, 2.91 mg mL1 nanorods or 4.07 mg mL1 nanocubes). And three colloidal solutions (5 mL) were then dropped on three pretreated GE surfaces and allowed to dry at 70 1C. Then, an aliquot of 2 mL of nafion solution (0.5 wt%) was cast on the layer of Co3O4 nanomaterials and dried at 70 1C. Cyclic voltammetric and amperometric measurements were performed on a CHI 660D (Chenhua, China). A three-electrode system comprising a platinum plate as auxiliary, a saturated calomel electrode (SCE) as reference and the Co3O4 nanomaterials-modified electrode as working electrodes was used for all electrochemical experiments.
3. Results and discussion 3.1. Characterization of Co3O4 nanomaterials with different shapes The morphology of the synthesized products was characterized by SEM and TEM. Fig. 1A and B show that the Co3O4 nanoplates are uniform and their average edge length and thickness are 100 nm and 21 nm, respectively. Fig. 1C and D show that the Co3O4 nanorods have a diameter of 10 nm and the length ranging from 40 nm to 80 nm. And Co3O4 nanocubes are monodispersed with a uniform edge of 20 nm (Fig. 1E and F). The crystallographic nature of the three Co3O4 nanomaterials was examined using high-resolution TEM (HRTEM) measurements.
Fig. 1 SEM images of Co3O4 nanoplates (A), Co3O4 nanorods (C) and Co3O4 nanocubes (E), TEM images of Co3O4 nanoplates (B), Co3O4 nanorods (D) and Co3O4 nanocubes (F).
This journal is © the Owner Societies 2014
Fig. 2 (A) TEM image of a typical Co3O4 nanoplate, HRTEM images of site 1 (B), site 2 (C) and site 3 (D) of Co3O4 nanoplate, (E) a HRTEM image of a Co3O4 nanorod, (F) a HRTEM image of a Co3O4 nanocube, and (G) exposed crystal planes on the nanoplate, nanorod, and nanocube of Co3O4.
The HRTEM images of selected sites 1, 2, and 3 of a typical Co3O4 nanoplate are shown in Fig. 2A–D. The dominant exposed planes of Co3O4 nanoplates are {112} planes, which are the only planes normal to both the set of (220) planes with a lattice spacing of 0.28 nm and the set of (222) planes with a square crossing lattice spacing of 0.23 nm.26 As is shown in Fig. 2E, both the set of (311) planes with a lattice spacing of 0.24 nm and set of (111) planes with a crossing lattice spacing of 0.467 nm are observed for Co3O4 nanorods. Therefore, the predominantly exposed planes of Co3O4 nanorods are {110} planes.37 The HRTEM image of Co3O4 nanocubes is shown in Fig. 2F. The dominant exposed planes of Co3O4 nanocubes are {100} planes, which are the only planes normal to the set of (400) planes with a lattice spacing of 0.20 nm.38 The crystal structures of the three Co3O4 nanomaterials were determined by X-ray powder diffraction (XRD). Despite variations in shape, all the patterns obtained can be indexed to the spinel structure of Co3O4 in good agreement with the reported data (JCPDS no. 76-1802) and no impurity peaks are observed, indicating the high purity of the final products (Fig. 3).39 The three Co3O4 nanomaterials were synthesized in the absence of organic macromolecular surfactants. The IR spectra of the three Co3O4 nanomaterials all display two sharp bands at 578 cm1 (n1) and 666 cm1 (n2), a broad band at 3409 cm1, and no other evident bands (Fig. S1, ESI†). The two sharp bands originate from the stretching vibrations of the metal–oxygen bond.40 The n1 band at 578 cm1 is characteristic of Co3+ vibration in the octahedral hole, and the n2 band at 666 cm1 is attributable to Co2+ vibration in the tetrahedral hole in the spinel lattice, confirming the formation of Co3O4 spinel oxide. The broad band at 3409 cm1 is assigned to the O–H stretching vibration due to the adsorbed H2O molecules.8 As a result, the products are free of coatings surrounding their surfaces. Therefore, the effect of coatings on their catalytic ability is excluded.
Phys. Chem. Chem. Phys., 2014, 16, 15709--15716 | 15711
View Article Online
Published on 30 May 2014. Downloaded by University of Tasmania on 31/08/2014 14:55:42.
Paper
PCCP
Fig. 3 XRD patterns of (a) Co3O4 nanoplates, (b) Co3O4 nanorods and (c) Co3O4 nanocubes.
The N2 adsorption–desorption isotherms of the three Co3O4 nanomaterials are shown in Fig. S2 (ESI†). The specific surface areas of Co3O4 nanoplates, nanorods and nanocubes are 97.1, 103.2 and 73.7 m2 g1, respectively. 3.2. The peroxidase-like catalytic activities of different Co3O4 nanomaterials The natural peroxidases can catalyze the oxidation of the substrates (amines and phenols) in the presence of H2O2 to develop the colour change.41 Therefore, the catalysis of TMB by the Co3O4 nanomaterials was tested in the presence of H2O2. In order to exclude the factor of surface area affecting the catalytic properties, the amount of each Co3O4 nanomaterials provides the same surface area in the reaction system.42 As shown in Fig. 4, in the absence of any Co3O4 nanomaterials, the solution containing TMB and H2O2 is colourless and has no absorption in the range from 500 to 800 nm. However, in the presence of any Co3O4 nanomaterials, the solutions produce a blue colour with absorbance maxima at 652 nm, but the absorbance intensity varies with the different shapes of
Fig. 4 The relative peroxidase-like activity of Co3O4 nanomaterials with TMB as the substrate. (1) Nanoplates; (2) nanorods; (3) nanocubes; and (4) in the absence of Co3O4 nanomaterials. The inset show the timedependent catalytic activity of Co3O4 nanomaterials with different shapes. The photograph showed the colour development of different samples after 5 min. The amount of each Co3O4 nanomaterials corresponded to a surface area of 3 103 m2. Experiments were carried out using 10.3 mg mL1 Co3O4 nanoplates (9.7 mg mL1 Co3O4 nanorods or 13.6 mg mL1 Co3O4 nanocubes), 0.5 mM TMB and 50 mM H2O2 in 3 mL phosphate buffer (100 mM, pH 6) and measured after 5 min using a UV-vis spectrometer. The time-dependent catalyses were measured at 652 nm.
15712 | Phys. Chem. Chem. Phys., 2014, 16, 15709--15716
Fig. 5 The relative peroxidase-like activity of Co3O4 nanomaterials with DAB (A) and OPD (B) as substrates. The insets show the time-dependent catalytic activity of Co3O4 nanomaterials with different shapes. The photographs showed the colour development of different samples after 15 min. (1) nanoplates; (2) nanorods; (3) nanocubes; and (4) in the absence of Co3O4 nanomaterials. Experiments were carried out using 10.3 mg mL1 Co3O4 nanoplates (9.7 mg mL1 Co3O4 nanorods or 13.6 mg mL1 Co3O4 nanocubes), 1 mM DAB and 200 mM H2O2 in 100 mM phosphate buffer (pH 4), or 10 mM OPD and 100 mM H2O2 in 100 mM phosphate buffer (pH 6), and then measured after 15 min using a UV-vis spectrometer. The timedependent catalyses were measured at 464 nm (DAB) and 436 nm (OPD).
Co3O4. The absorption originates from the oxidation of TMB, similar to the phenomenon observed for horseradish peroxidase (HRP).3 And the difference in absorbance intensity is attributed to the different catalytic activities of Co3O4 nanomaterials. The inset of Fig. 4 also shows the time-dependent catalytic activity of different Co3O4 nanomaterials under the standard reaction conditions. The different Co3O4 nanomaterials exhibit different levels of activity over the reaction time, following the order of nanoplates c nanorods 4 nanocubes. To further characterize the different peroxidase-like catalytic activities of the Co3O4 nanomaterials, the experiments were conducted using other usually used peroxidase substrates in place of TMB, including DAB and OPD. As shown in Fig. 5A, with any of the Co3O4 nanomaterials, the solutions of DAB and H2O2 display a brown colour, which originates from the oxidation product of DAB, but there is still a difference in absorbance intensity for different Co3O4 nanomaterials. A similar catalytic performance is observed when OPD is used as the substrate (Fig. 5B). The three results all indicate that different Co3O4 nanomaterials own different levels of catalytic activity, in the order of nanoplates c nanorods 4 nanocubes. To exclude the difference in their catalytic activity resulting from the different leaching of cobalt ions, the Co3O4 nanomaterials were incubated in reaction buffer (pH 6.0) for 30 min
This journal is © the Owner Societies 2014
View Article Online
PCCP
Paper
and then removed by centrifugation. The catalytic activities of the leaching solutions were tested under standard conditions. All of the leaching solutions have a negligible absorption (data not shown), indicating that their catalytic activities are derived from the intact Co3O4 nanomaterials themselves rather than as a result of leaching cobalt ions.
Published on 30 May 2014. Downloaded by University of Tasmania on 31/08/2014 14:55:42.
3.3. Effect of external conditions on the catalytic activity of the three Co3O4 nanomaterials The substrates TMB and H2O2 are selected as the model systems to investigate the catalytic properties of Co3O4 nanomaterials with different shapes. In order to understand the effect of external conditions on the catalytic ability of the Co3O4 nanomaterials, their catalytic activities were measured by varying the pH from 2 to 8 and the temperature from 20 to 50 1C. Fig. 6 shows that the optimal pH is 6, indicating that the oxidation of TMB occurs easily under weakly acidic conditions, as have also been reported for HRP. And the optimal temperature is 25 1C, exhibiting the high catalytic ability even at room temperature. All the Co3O4 nanomaterials as peroxidase mimics have the same optimal pH and temperature, thus, pH 6 and 25 1C are adopted as standard reaction conditions for the subsequent study of their catalytic properties. The above results indicate that the different catalytic activities of Co3O4 nanomaterials are not associated with the external conditions but their different shapes. 3.4.
The kinetic study of Co3O4 nanomaterials
kinetic data were measured by varying the concentration of one substrate while keeping the concentration of the other substrate constant. At a fixed concentration of one substrate, typical Michaelis–Menten curves are observed for Co3O4 nanoplates, Co3O4 nanorods and Co3O4 nanocubes (Fig. 7). The Lineweaver–Burk double reciprocal plots (Fig. S3, ESI†) show good linear relationship between v1 and [S]1. The Vmax and Km of the three Co3O4 nanomaterials could be obtained by the slopes and intercepts of these lines. The experimental data are well fitted to the Michaelis–Menten equation to obtain the parameters shown in Table 1. These results indicate that the reaction catalyzed by the three Co3O4 nanomaterials all follow the typical Michaelis–Menten model. The Vmax values with H2O2 and TMB as the two substrates all increase from nanocubes through nanorods to nanoplates, showing the highest maximal reaction velocity. On the other hand, the lowest Km values with the two substrates are shown with the nanoplates followed by the nanorods and then the nanocubes. Vmax values are the indicators of reaction activity (i.e. the rate of reaction when an enzyme is saturated with the substrate). The Vmax values of the three Co3O4 nanomaterials show that nanoplates have the highest peroxidase-like catalytic activity and nanorods exhibit medium activity, higher than nanocubes, which is consistent with the above results. Km values represent the affinity of the enzyme towards the substrate. The lower the Km values, the greater the affinity. The Km values of the three Co3O4 nanomaterials suggest that nanoplates have the highest affinity, nanorods ranked the second place, the Km values of
The peroxidase-like catalytic properties of the Co3O4 nanomaterials were further investigated using steady-state kinetics. Co3O4 nanomaterials catalyzed the bisubstrate reaction, so the
Fig. 6 Effects of pH (A) and temperature (B) on the catalytic activity of Co3O4 nanoplates (’), Co3O4 nanorods (K) and Co3O4 nanocubes (m). Experiments were carried out using 10.3 mg mL1 Co3O4 nanoplates (9.7 mg mL1 Co3O4 nanorods or 13.6 mg mL1 Co3O4 nanocubes), 0.5 mM TMB and 50 mM H2O2. (A) 100 mM Na2HPO4-acetic buffers of different pHs. (B) 100 mM phosphate buffer (pH 6). The maximum point in each curve was set to 100%.
This journal is © the Owner Societies 2014
Fig. 7 Steady-state kinetic assay. The velocity of the reaction was measured using 10.3 mg mL1 Co3O4 nanoplates (9.7 mg mL1 Co3O4 nanorods or 13.6 mg mL1 Co3O4 nanocubes) in 100 mM phosphate buffer (pH 6.0). (A), (C) and (E) The concentration of H2O2 was 50 mM and the TMB concentration was varied. (B), (D) and (F) The concentration of TMB was 0.5 mM and the H2O2 concentration was varied.
Phys. Chem. Chem. Phys., 2014, 16, 15709--15716 | 15713
View Article Online
Paper
Published on 30 May 2014. Downloaded by University of Tasmania on 31/08/2014 14:55:42.
Table 1
PCCP Comparison of morphological features and kinetic parameters of various Co3O4 nanomaterials and HRP
Surface area/m2 g1
Substrates
Km/mM
Vmax/108 M s1
Kcat/103 s1
0.90
TMB H2O2
0.090 284
9.91 48.08
11.01 53.42
103.2
44.29
TMB H2O2
0.22 455
7.12 39.60
0.16 0.89
73.7
46.50
TMB H2O2
0.26 480
6.04 35.81
0.13 0.77
2.50
TMB H2O2
0.43 3.70
10.00 8.71
4.00 3.48
Catalyst
Exposed planes
Size/nm
Nanoplates
{112}
21 (thickness) 100 (edge)
97.1
Nanorods
{110}
10 (diameter) 40–80 (length)
Nanocubes
{100}
20 (edge)
HRPa a
[E]/1011 M
Obtained by the previous report.3
both are higher than that of nanocubes. This may be due to the presence of more ‘‘active sites’’ on the surface of the Co3O4 nanoplates compared to nanorods and nanocubes, and the substrates are able to coordinate more easily with the metal centers.8 Therefore, the Co3O4 nanoplates exhibit the highest affinity and catalytic activity. In order to compare the catalytic activities of Co3O4 nanomaterials and natural peroxidase, their Kcat values (catalytic constants) were obtained. The Kcat values of Co3O4 nanorods and nanocubes are smaller than those of HRP, indicating that the catalytic activities of Co3O4 nanorods and nanocubes are lower than that of HRP. However, the increase of catalytic activities from HRP to Co3O4 nanoplates is by one order of magnitude based on their Kcat values. The results demonstrate that shape-controllable Co3O4 nanomaterials can be highly efficient peroxidase mimics. 3.5. Electrocatalytic activity of Co3O4 nanomaterials toward reduction of H2O2 The direct electrochemistry of heme-containing enzymes at electrodes has been well studied in order to understand the electron-transfer process of enzymes.43 It has been shown that the catalytic activity of some nanoparticles as enzyme mimics originated from their electron transfer ability.4,8 Therefore, the electrocatalytic behaviour of the Co3O4 nanomaterials modified electrode towards the reduction of H2O2 was studied using cyclic voltammetry and amperometric responses. In the presence of H2O2, no obvious current is found with bare GE, but evident peaks of reduction at 0.55 V are observed using the Co3O4 nanomaterials modified GE (Fig. 8A). The reduction currents at 0.55 V increase steeply to reach a steady-state value with addition of an aliquot of H2O2 (Fig. 8B). The three Co3O4 nanomaterials modified GE show different reduction currents, following the order of nanoplates c nanorods 4 nanocubes. The results demonstrate clearly that three Co3O4 nanomaterials exhibit different electrocatalytic activities towards the reduction of H2O2, which indicate that they have the different abilities of electron transfer between the electrode (electron donor) and H2O2 (electron acceptor). The {100} plane contains only one Co2+ cation which is not favourable for its electron transfer (Fig. 9). There are both Co2+ cations and Co3+ cations on {110} and {112} planes (Fig. 9), so their electron transfer abilities are higher than that of the {100} plane. The {112} plane exhibits the
15714 | Phys. Chem. Chem. Phys., 2014, 16, 15709--15716
Fig. 8 (A) Cyclic voltammograms of the Co3O4 nanomaterials modified GE electrodes, 40 mM H2O2. (B) Amperometric response of the Co3O4 nanomaterials modified GE at the applied potential of 0.55 V upon successive addition of 2.5 mM H2O2. Nanoplates (1), nanorods (2), nanocubes modified GE (3) and bare GE (4), 100 mM phosphate buffer (pH 6).
highest electron transfer, which arises from its particular atom arrangement. In one word, the three crystal planes have different arrangements of surface atoms, thus exhibiting different electron transfer abilities, which induce their different peroxidaselike catalytic activities. 3.6. Crystal plane effect on the catalytic activities of Co3O4 nanomaterials The above results showed that the three kinds of Co3O4 nanomaterials exhibit obviously different peroxidase-like catalytic activities. Co3O4 nanoplates with {112} planes exhibit not only the highest catalytic rate but also the best affinity towards the substrates. The catalytic activity of the three Co3O4 nanomaterials is ranked in the order ‘‘nanoplates ({112} planes) c nanorods ({110} planes) 4 nanocubes ({100} planes)’’, which could be largely attributed to the ‘‘crystal plane’’ effect. Regarding the same surface areas of the three kinds of Co3O4
This journal is © the Owner Societies 2014
View Article Online
Published on 30 May 2014. Downloaded by University of Tasmania on 31/08/2014 14:55:42.
PCCP
Fig. 9 The surface atomic configurations in the {112}, {110} and {100} planes of the Co3O4 crystal. The Co3O4 Crystallographic Information File (CIF) was taken from the NIST/FIZ Inorganic Crystal Structure Database. The surface atomic configurations were drawn by the CIF and the software of Materials Studio.26
nanomaterials in the reaction systems, this further confirms that the differences in their peroxidase-like catalytic activity originate from the crystal plane effect and the well-defined crystal plane is the dominant factor in Co3O4 nanomaterials. The surface atomic configurations in the {112}, {110} and {100} planes of the Co3O4 unit cell are shown in Fig. 9. According to the surface atomic configuration in a unit cell, the surface layer of the {100} plane contains one Co2+ cation on average and does not contain Co3+ cations, but there are two Co3+ cations in the sub-layer. It has been explained that the catalytic activity of metal oxide NPs originated from the ion pairs (catalytic sites in two different oxidation states).8,44 So it is difficult for the substrates to come into contact with the Co3+ cations during the catalyzed cycle by Co3O4 nanocubes. The Co3+ cations on the sub-layers are exposed possibly in the defects, and these defects are usually the active sites, which contribute considerably to the catalysis. Therefore, Co3O4 nanocubes with {100} planes exhibit lower catalytic activity than the other two Co3O4 nanomaterials. As illustrated in the atomic configurations (Fig. 9), unlike Co3O4 nanocubes with {100} planes, there are not only Co2+ cations but also Co3+ cations in Co3O4 nanorods with {110} planes and Co3O4 nanoplates with {112} planes. Although both the {110} and {112} planes contain two Co2+ cations and two Co3+ cations, respectively, their arrangement manner of surface atoms is different (as shown in Fig. 9). The high-index plane of {112} possesses a more open structure than that of the {110} plane, and its density of stepped atoms is higher than that of the {110} plane (Fig. 9). Therefore, the {112} plane has a more reactive surface which leads to the highest catalytic ability in the three planes.
4. Conclusions Co3O4 nanomaterials with different shapes including nanoplates, nanorods and nanocubes, were synthesized. The different
This journal is © the Owner Societies 2014
Paper
Co3O4 nanomaterials expose different crystal planes, and the predominantly exposed planes of nanoplates, nanorods and nanocubes are {112}, {110} and {100} planes, respectively. They exhibit different peroxidase-like catalytic activities, in the order of nanoplates c nanorods 4 nanocubes. The catalytic performances all follow the typical Michaelis–Menten model. The Vmax values show that nanoplates have the highest peroxidase-like catalytic activity and nanorods exhibit medium activity, higher than nanocubes. The Km values suggest that nanoplates have the highest affinity towards the substrate (TMB and H2O2), nanorods ranked the second place, the Km values of both are higher than that of nanocubes. Compared with natural peroxidase, the catalytic activity of Co3O4 nanoplates is even higher than that of HRP. The three Co3O4 nanomaterials as peroxidase mimics have the same optimal pH and temperature, indicating that their different catalytic activities are not associated with the external conditions but their different crystal planes. Therefore, the crystal plane of Co3O4 has a significant influence on their catalytic activities. The catalytic activity of the three crystal planes is ranked as: {112} c {110} 4 {100}. Their different catalytic activities depend greatly upon the different arrangements of surface atoms. The three crystal planes exhibit different electron transfer abilities, which induce the different peroxidaselike catalytic activities of Co3O4 nanomaterials. This study shows that the crystal planes obtained by shape control of Co3O4 nanomaterials can regulate the peroxidase-like catalytic activity. This suggests that we can design and synthesize the nanomaterials with well-defined crystal planes to improve their catalytic activity. The concept of crystal plane-dependent nanocatalysis not only enables the fundamental understanding of the catalytic phenomenon but also highlights the essential implications for the design and preparation of more efficient catalysts.
Acknowledgements The authors acknowledge the financial support from National Natural Science Foundation of China under the project number 21273057.
Notes and references 1 R. Breslow, Acc. Chem. Res., 1995, 28, 146–153. 2 Y. Murakami, J. Kikuchi, Y. Hisaeda and O. Hayashida, Chem. Rev., 1996, 96, 721–758. 3 L. Gao, J. Zhuang, L. Nie, J. Zhang, Y. Zhang, N. Gu, T. Wang, J. Feng, D. Yang, S. Perrett and X. Yan, Nat. Nanotechnol., 2007, 2, 577–583. 4 Y. Song, K. Qu, C. Zhao, J. Ren and X. Qu, Adv. Mater., 2010, 22, 2206–2210. 5 W. He, X. Wu, J. Liu, X. Hu, K. Zhang, S. Hou, W. Zhou and S. Xie, Chem. Mater., 2010, 22, 2988–2994. ´, F. Nata ´lio, M. Humanes, J. Leppin, K. Heinze, 6 R. Andre ¨der, W. E. Mu ¨ller and W. Tremel, R. Wever, H. C. Schro Adv. Funct. Mater., 2011, 21, 501–509.
Phys. Chem. Chem. Phys., 2014, 16, 15709--15716 | 15715
View Article Online
Published on 30 May 2014. Downloaded by University of Tasmania on 31/08/2014 14:55:42.
Paper
7 H. Deng, W. Shen, Y. Peng, X. Chen, G. Yi and Z. Gao, Chem. – Eur. J., 2012, 18, 8906–8911. 8 J. Mu, Y. Wang, M. Zhao and L. Zhang, Chem. Commun., 2012, 48, 2540–2542. 9 J. Mu, L. Zhang, M. Zhao and Y. Wang, J. Mol. Catal. A: Chem., 2013, 378, 30–37. 10 E. G. Heckert, A. S. Karakoti, S. Seal and W. T. Self, Biomaterials, 2008, 29, 2705–2709. 11 T. Pirmohamed, J. M. Dowding, S. Singh, B. Wasserman, E. Heckert, A. S. Karakoti, J. E. S. King, S. Seal and W. T. Self, Chem. Commun., 2010, 46, 2736–2738. 12 C.-W. Tseng, H.-Y. Chang, J.-Y. Chang and C.-C. Huang, Nanoscale, 2012, 4, 6823–6830. 13 W. Chen, J. Chen, A. L. Liu, L. M. Wang, G. W. Li and X. H. Lin, ChemCatChem, 2011, 3, 1151–1154. 14 L. Gao, J. Wu, S. Lyle, K. Zehr, L. Cao and D. Gao, J. Phys. Chem. C, 2008, 112, 17357–17361. ¨gl and S. B. Abd Hamid, Angew. Chem., Int. Ed., 15 R. Schlo 2004, 43, 1628–1637. 16 M. Valden, X. Lai and D. W. Goodman, Science, 1998, 281, 1647–1650. ¨ckner, S. Zhang and 17 F. Shi, M. K. Tse, M. M. Pohl, A. Bru M. Beller, Angew. Chem., Int. Ed., 2007, 46, 8866–8868. 18 N. Spencer, R. Schoonmaker and G. Somorjai, Nature, 1981, 294, 643–644. 19 N. Spencer, R. Schoonmaker and G. Somorjai, J. Catal., 1982, 74, 129–135. 20 J. A. van Bokhoven, ChemCatChem, 2009, 1, 363–364. 21 B. Lim, M. Jiang, J. Tao, P. H. Camargo, Y. Zhu and Y. Xia, Adv. Funct. Mater., 2009, 19, 189–200. 22 B. Lim, J. Wang, P. H. Camargo, C. M. Cobley, M. J. Kim and Y. Xia, Angew. Chem., Int. Ed., 2009, 48, 6304–6308. 23 Y. w. Jun, J. s. Choi and J. Cheon, Angew. Chem., Int. Ed., 2006, 45, 3414–3439. 24 S. Agarwal, L. Lefferts, B. L. Mojet, D. Ligthart, E. J. Hensen, D. R. Mitchell, W. J. Erasmus, B. G. Anderson, E. J. Olivier and J. H. Neethling, ChemSusChem, 2013, 6, 1898–1906. 25 Z. A. Qiao, Z. Wu and S. Dai, ChemSusChem, 2013, 6, 1821–1833.
15716 | Phys. Chem. Chem. Phys., 2014, 16, 15709--15716
PCCP
26 L. Hu, Q. Peng and Y. Li, J. Am. Chem. Soc., 2008, 130, 16136–16137. 27 X. Xie, Y. Li, Z.-Q. Liu, M. Haruta and W. Shen, Nature, 2009, 458, 746–749. 28 E. Aneggi, D. Wiater, C. de Leitenburg, J. Llorca and A. Trovarelli, ACS Catal., 2014, 4, 172–181. 29 I. Lee, F. Delbecq, R. Morales, M. A. Albiter and F. Zaera, Nat. Mater., 2009, 8, 132–138. 30 N. Tian, Z.-Y. Zhou, S.-G. Sun, Y. Ding and Z. L. Wang, Science, 2007, 316, 732–735. 31 L. Xiao, L. Zhuang, Y. Liu and J. Lu, J. Am. Chem. Soc., 2008, 131, 602–608. 32 G. Centi and S. Perathoner, Eur. J. Inorg. Chem., 2009, 3851–3878. 33 J. Zeng, Q. Zhang, J. Chen and Y. Xia, Nano Lett., 2009, 10, 30–35. 34 D.-e. Jiang and S. Dai, Phys. Chem. Chem. Phys., 2011, 13, 978–984. 35 S. Hwang, A. Umar, S. Kim, S. Al-Sayari, M. Abaker, A. Al-Hajry and A. M. Stephan, Electrochim. Acta, 2011, 56, 8534–8538. 36 X. C. Song, X. Wang, Y. F. Zheng, R. Ma and H. Y. Yin, J. Nanopart. Res., 2011, 13, 1319–1324. 37 Z. Fei, S. He, L. Li, W. Ji and C.-T. Au, Chem. Commun., 2012, 48, 853–855. 38 R. Xu and H. C. Zeng, Langmuir, 2004, 20, 9780–9790. 39 B. Varghese, C. Teo, Y. Zhu, M. V. Reddy, B. V. Chowdari, A. T. S. Wee, V. Tan, C. T. Lim and C. H. Sow, Adv. Funct. Mater., 2007, 17, 1932–1939. 40 J. Yin, H. Cao and Y. Lu, J. Mater. Chem., 2012, 22, 527–534. 41 A. M. Klibanov, T.-m. Tu and K. P. Scott, Science, 1983, 221, 259–261. 42 Y. Teng, Y. Kusano, M. Azuma, M. Haruta and Y. Shimakawa, Catal. Sci. Technol., 2011, 1, 920–922. 43 L. Gorton, A. Lindgren, T. Larsson, F. Munteanu, T. Ruzgas and I. Gazaryan, Anal. Chim. Acta, 1999, 400, 91–108. 44 I. E. Wachs, Catal. Today, 2005, 100, 79–94.
This journal is © the Owner Societies 2014
